# $subtract (aggregation)¶

## Definition¶

`$subtract`

¶Subtracts two numbers to return the difference, or two dates to return the difference in milliseconds, or a date and a number in milliseconds to return the resulting date.

The

`$subtract`

expression has the following syntax:{ $subtract: [ <expression1>, <expression2> ] } The second argument is subtracted from the first argument.

The arguments can be any valid expression as long as they resolve to numbers and/or dates. To subtract a number from a date, the date must be the first argument. For more information on expressions, see Expressions.

## Examples¶

Consider a `sales`

collection with the following documents:

db.sales.insertMany([ { "_id" : 1, "item" : "abc", "price" : 10, "fee" : 2, "discount" : 5, "date" : ISODate("2014-03-01T08:00:00Z") }, { "_id" : 2, "item" : "jkl", "price" : 20, "fee" : 1, "discount" : 2, "date" : ISODate("2014-03-01T09:00:00Z") } ])

### Subtract Numbers¶

The following aggregation uses the `$subtract`

expression
to compute the `total`

by subtracting the `discount`

from the
subtotal of `price`

and `fee`

.

db.sales.aggregate( [ { $project: { item: 1, total: { $subtract: [ { $add: [ "$price", "$fee" ] }, "$discount" ] } } } ] )

The operation returns the following results:

{ "_id" : 1, "item" : "abc", "total" : 7 } { "_id" : 2, "item" : "jkl", "total" : 19 }

### Subtract Two Dates¶

The following aggregation uses the `$subtract`

expression
to subtract `$date`

from the current date, using the system
`NOW`

(available starting in 4.2) and returns the difference in
milliseconds:

db.sales.aggregate( [ { $project: { item: 1, dateDifference: { $subtract: [ "$$NOW", "$date" ] } } } ] )

Alternatively, you can use the `Date()`

for the current date:s

db.sales.aggregate( [ { $project: { item: 1, dateDifference: { $subtract: [ new Date(), "$date" ] } } } ] )

Both operations return documents that resemble the following:

{ "_id" : 1, "item" : "abc", "dateDifference" : NumberLong("186136746187") } { "_id" : 2, "item" : "jkl", "dateDifference" : NumberLong("186133146187") }

### Subtract Milliseconds from a Date¶

The following aggregation uses the `$subtract`

expression
to subtract 5 * 60 * 1000 milliseconds (5 minutes) from the "$date"
field:

db.sales.aggregate( [ { $project: { item: 1, dateDifference: { $subtract: [ "$date", 5 * 60 * 1000 ] } } } ] )

The operation returns the following results:

{ "_id" : 1, "item" : "abc", "dateDifference" : ISODate("2014-03-01T07:55:00Z") } { "_id" : 2, "item" : "jkl", "dateDifference" : ISODate("2014-03-01T08:55:00Z") }